The present invention relates to devices and methods for sensing pressure, and more particularly to vacuum-coupled and gas-volume-coupled pressure sensing systems and their methods of use.
Pressure measurements are attained by determining the magnitude of a force that is applied to a unit area. A pressure sensor is a device that reads changes in the force applied to the unit area and transmits indicia of such changes in force to another apparatus. The transmitted indicia of changes in force may be converted to an electrical, mechanical, or pneumatic parameter and may provide a display or reading of the pressure.
Pressure sensors may use various means for sensing and transmitting indicia of force per unit area. Some common types of pressure sensors include simple fluid-filled columns to pistons, deflecting members, strain gauges, semiconductor piezoresistive apparatus, piezoelectric sensors (including dynamic and quasistatic measurement types), microelectromechanical systems (MEMS), vibrating elements, optical pressure sensors and variable capacitance systems. Depending on the type of pressure sensor employed, the indicia of changes in force per the unit area my be transmitted from the pressure receiving pressure sensor to a pressure transducer in various forms, including the transmission of pressure waves through a fluid-filled column, transmission of electromagnetic signals through a wire or wireless connection or transmission of optical signals through optical fibers.
In some instances it is desirable to disallow direct contact between the fluid or other matter in which pressure is to be sensed and some pressure receiving surface(s) of the pressure sensor, transducer or other apparatus in the system. For example, in cases where one desires to measure changes in the pressure of a sterile fluid being pumped through a pump housing or conduit, it will likely be desirable to avoid direct contact between that sterile fluid whose pressure is being measured and any non-sterile components of a pressure sensor and/or transducer. Similarly, in cases where it is desired to measure the pressure of a corrosive or potentially damaging fluid (e.g., an acid) being pumped or fed through a pipeline it may be desirable to avoid direct contact between that fluid and any metal or other surfaces within the pressure sensor or transducer that my become corroded or damaged by the corrosive or otherwise damaging fluid. Or, in cases where it is desired to measure the pressure of blood flowing through an artery or vein of a human or veterinary patient, it is clearly desirable to avoid direct contact between the patient""s blood and any non-sterile or potentially toxic components of the pressure sensor or transducer.
Accordingly, there is a need for the development of barriers for a) preventing direct contact between the fluid or other matter in which the pressure is to be sensed and the pressure receiving surfaces of the pressure sensor, transducer or other apparatus used for obtaining the desired pressure measurement while, at the same time, b) allowing accurate transmission of the pressure changes or other indicia of changes in force per unit area to be transmitted to the pressure receiving surfaces of the pressure sensor, transducer or other apparatus without undue damping, distortion or disruption. Although placement of an interposed flexible film or membrane may serve to prevent direct contact between the fluid or other matter in which the pressure is being measured and the pressure receiving surfaces of the pressure sensor, transducer or other apparatus, the use of such flexible film or membrane may be problematic in systems where the pressure(s) being sensed is/are at least some of the time negative or below ambient, as the presence of such negative or sub-ambient pressure on one side of the film or membrane may tend to pull the film or membrane away from the pressure sensing surface, thereby interfering with or preventing accurate measurement of the negative or sub-ambient pressure.
As those of skill in the art will recognize, certain medical procedures are performed in which fluid irrigation and aspiration loops are superimposed upon a generally enclosed surgical sight for the purposes of maintaining visibility and/or removing tissue and/or debris from the surgical field. For example, for ophthalmic procedures in the eye, irrigation flow and inflation of the eye is often achieved by allowing an irrigation fluid (e.g., basic salt solution) to undergo gravity flow from an elevated bottle or bag containing the irrigation fluid, through tubing lines to the eye. An aspiration apparatus such as a peristaltic pump or other aspiration type pump may be used to aspirate the aspirant from the eye through another tube. It is often desirable to accurately determine the pressure of the aspirant fluid within the housing of the aspiration pump or in the tube that runs from the eye to the aspiration pump. However, especially when the pressure is measured within the pump, such pressure may at least some of the time be negative (e.g., less than the ambient atmospheric pressure). Thus, the pressure sensor used in such application must be capable of sensing negative (as well as positive) pressure. Furthermore, it is desirable for the irrigation solution to be sterile and free of contaminants. Thus, if the pressure sensing device contains a non-sterile or contaminated pressure receiving surface, it will be desirable to prevent the aspirant from coming in direct contact with the pressure receiving surface of the pressure sensor.
The present invention overcomes at least some of the shortcomings of the prior art by providing pressure measuring devices and methods wherein flexible barriers prevent direct contact between the fluid or other matter in which the pressure is being measured and the pressure receiving surfaces of the pressure sensing devices (e.g., transducers, pressure switches, pistons, deflecting members, strain gauges, semiconductor piezoresistive apparatus, piezoelectric sensors (including dynamic and quasistatic measurement types), microelectromechanical systems (MEMS), vibrating elements (silicon resonance, for example), optical pressure sensors, variable capacitance systems or other pressure measuring apparatus). One example of a particular pressure sensing apparatus that may be used in at least some embodiments of the invention is that commercially available under the name SenSym Type 19c from InvEnsys Sensor Systems, Milpitas, Calif. The flexible barriers are coupled to the pressure receiving surfaces of the sensing apparatus such that the pressure exerted against the flexible barrier is transmitted to and accurately received by the pressure sensing apparatus even in situations where the pressure exerted against the flexible barrier is negative as would tend to pull or separate the flexible barrier from the pressure sensing apparatus. This coupling of the flexible barrier to the pressure sensing apparatus may be accomplished by various means including the creation of a vacuum between the flexible barrier and the pressure sensing apparatus, the application of an adhesive between the flexible barrier and the pressure sensing apparatus, the capturing of a fixed volume of fluid (e.g, liquid or suitable) between the flexible barrier and the pressure sensing apparatus or the exclusion of all or all but a small amount of air from the space between the flexible barrier and the pressure sensing apparatus and the sealing of such space to prevent subsequent inflow of fluid between the flexible barrier and the pressure sensing apparatus. Thus, the pressure sensors of this invention are useable to measure negative or sub-ambient as well as positive fluid pressure.
Further in accordance with the invention, in at least some embodiments and particularly in those in embodiments where the pressure sensing system is being used to obtain dynamic pressure measurements (e.g., continuous pressure monitoring, monitoring of a pressure waveform or tracing and/or rapidly sensing changes in pressure), it is desirable to minimize the compliance of all components of the system including the flexible barrier member and any gas, liquid, adhesive material or other matter interposed between the flexible barrier and the pressure sensing apparatus. Accordingly, in some embodiments of this invention, the material and construction of the flexible barrier and the means by which the flexible barrier is coupled to the pressure receiving surface of the pressure sensing apparatus may be made as non-compliant as possible, thereby avoiding possible damping or attenuation of the pressure measurement due to excessive system compliance. In this regard, those embodiments in which the flexible membrane is affixed directly to and is maintained in abutting and continuous contact with the pressure receiving surface of the pressure sensing apparatus (e.g., where the flexible barrier is coupled to the pressure sensing apparatus by vacuum or thin layer of adhesive) will likely be of relatively minimal compliance and may be better suited to applications wherein the system is used for such dynamic pressure monitoring. Other embodiments wherein the flexible barrier is more compliant, or where some fluid-filled or compressible-matter-containing space exists between the flexible barrier and the pressure sensing apparatus may be of higher compliance and may be more suited to static pressure monitoring or applications where in rapid or instantaneous pressure measurements are not required (e.g., measuring process pressures that do not change rapidly, measuring mean pressure, etc.
Further in accordance with this invention, some of the pressure sensing systems provided comprise a disposable portion and a reusable portion, wherein the flexible barrier member prevents direct contact between the reusable portion (which may be contaminated or potentially toxic/damaging to the fluid) and the fluid in which pressure is being measured. Thus, such pressure sensing devices are useable in applications such as medical pumping devices, etc. wherein it is desired to measure the pressure of a fluid that is sterile or which otherwise would be damaged or changed if it were to come into direct contact with the pressure receiving surface(s) of the pressure sensing apparatus.
These general aspects of the invention, as well as numerous other aspects and advantages of the invention, will become apparent to persons of skill in the art who read and understand the following detailed description and the accompanying drawings.